Dehydrogenation method



ed States 3,0883% DEHYDR6GENATIN METHOD Donald H. Stevenson, MilrnontPark, Pa., assignor to Arr Products and Chemicals, Inc., a corporationof Delaware No Drawing. Filed June 17, 1959, Ser. No. 820,849 1 Claim.(Cl. 260680) This invention relates to the dehydrogenation of a mixtureconsisting essentially of C and C hydrocarbons. The term consistingessentially of as used herein is intended to exclude the presence ofreactants significantly affecting the products, but to permit thepresence of the minor amounts of impurities ordinarily present in thecommercially employed compositions of an analogous nature. For example,a recycle stream from a hydrocarbon dehydrogenation may contain theminor amounts of lower hydrocarbons and/or higher hydrocarbons becausethe separation procedures are intended to remove such by-productscheaply and are not intended to reduce the impurity levels to those ofultra pure materials.

Heretofore, various proposals have been made for the preparation ofbutadiene by the cracking of normal pentane and by the catalyticdehydrogenation of normal butane. It has been proposed that a catalystbe employed for dehydrogenating hydrocarbons of four carbon atoms untilpartially deactivated, and that the partially deactivated catalyst beemployed for dehydrogenatin-g hydrocarbons of five carbon atoms. Suchblocked out operation has permitted the separation of the desiredproducts from relatively simple mixtures. Similarly, the use of thecombination of entirely separate dehydrogenation units and entirelyseparate recovery systems for the C hydrocarbons and the C hydrocarbonshas been favored for engineering reasons. It has long been recognizedthat the catalytic dehydrogenation of a mixture of C and C hydrocarbonswould produce an efiluent containing many components. Previously, it hasbeen known that as the number of components present in the efiiuent fromthe catalyst zone is increased, the difficulties in recovering goodyields of high purity products from the effluent from a catalyst chamberare increased much more than proportionately to the number ofcomponents. If one starts with a mixture of C and C hydrocarbons, and ifone desires to obtain as one product a high purity unsaturated Chydrocarbon and as another product a high purity unsaturated Chydrocarbon, then the separation costs are much lower if the fractionsare separately dehydrogenated after the relatively simple feedstockmixture is separated into a 0., fraction and a C fraction than if a highpurity unsaturated C hydrocarbon fraction and a high purity unsaturatedC hydrocarbon fraction are isolated from the extremely complex efiluentfrom a zone for the catalytic dehydrogenation of a mixture of C and Chydrocarbons. Accordingly, those familiar with the economics of suchproduction of high purity unsaturated hydrocarbon having fewer than 6carbon atoms by dehydrogenation (and with minimized cracking ofhydrocarbons of the same number of carbon atoms) had reason to discreditthe several suggestions indicating that it might be possible to subjecta mixture of C and C hydrocarbons to a dehydrogenation zone. The priorart literature pertinent to the catalytic dehydrogenation of mixtures ofC and C hydrocarbons taught those skilled in the art that the concept ofdehydrogenating a mixture of C and C hydrocarbons had been described,but that such a procedure was quite unattractive because the potentialadvantages from such mixed dehydrogenation were insuflicient tocompensate for the difficulties of recovering pure products from thecomplex mixture.

If the amount and composition of a recycle stream for an establishedcontinuous conversion of a gas stream are stabilized and known and ifthe calculations concern merely the fresh feed and the withdrawnproducts, then the recycle stream can be ignored'in such calculations.In research runs, however, the recycle stream (or the portion simulatingrecycle) is desirably deemed not only a part of the feedstock but also apart of the crude product stream, and the calculations are desirablybased upon such classifications. To the extent that the total iso Cfeedstock is not recovered as unreacted iso C hydrocarbons, it is deemedconverted to a mixture of desired products and less desired by-products.If the feedstock contains more isopentenes than the product, then thethus consumed isopentenes are a part of the iso C hydrocarbons whichdisappear and/or are converted. The extent of iso C conversion is thusequivalent to the disappearance of total iso C feed. The amount ofisoprene per pass represents a particular percentage of the convertediso C feed for that run, and such percentage is designated as theultimate isoprene selectivity if isopentenes are consumed, and asisoprene selectivity if there is a net production of isopentenes. Ifthere is a net production of isopentenes, then the amount thereof can beadded to the amount of isoprene and such sum constitutes the totaldesired iso C unsaturates per pass. The amount of total desired iso Cunsaturates per pass represents a particular percentage of the convertediso C feed, and such percentage is designated as total selectivity. Suchrequirements for selectivities can be expressed by formulae, such as:

Ultimate isoprene selectivity Isoprene per pass Disappearance ofisopentenes and isopentane Total selectivity Isoprene plus isopentenesper pass Disappearance of total iso C feed In accordance with thepresent invention, isoprene is prepared from a gas mixture containingisopentane as the predominant C hydrocarbon but the mixture may containrecycle C hydrocarbons comprising isopentenes. Said gas mixture is alsodesignated as an isopentane stream containing not more than 49%isopentenes. The isoprene is prepared with advantageous selectivity, andsimultaneously butadiene is prepared from a gas mixture containingnormal butane as the most abundant C hydrocarbon but the mixture maycontain normal butenes. Said gas mixture is also designated as a normalbutane stream containing not more than 49% normal butenes. For example,the recycle of C hydrocarbons may contain butenes. Such dehydrogenationof the C hydrocarbons produces advantageously small amounts ofby-products. A hydrocarbon gaseous mixture (preferably includingrecycled C C hydrocarbons) consisting essentially of from 10% to 60% byweight (8.2% to 54.7% by volume) iso C hydrocarbons (predominantlyisopentane) and from 40% to by weight (45.3% to 91.8% by volume) normalC hydrocarbons (normal butane being the most abundant C hydrocarbon) isdirected through a catalytic dehydrogenation zone ata space rate ofabout 1 to 6 liquid volumes per catalyst volume per hour at atemperature within the range from 1050 F. and 1200 F. at an absolutepressure less than 15 inches of mercury at a severity converting atleast 10% of the iso C hydrocarbons in a dehydrogenation-regenerationcycle maintaining adiabatic dehydrogenation conditions. The gaseousefliuent from the catalytic zone contains butenes (1- butene;Z-cisbutene; and Z-transbutene), butadiene, isopentenes (Z-methyll-butene; 3-methyl l-butene; and 2- methyl Z-butene), and isoprene(Z-methyl 1,3-butadiene). The selectivity for the formation of isoprenein the catalytic zone is superior to that attainable at equivalentconditions in the absence of the C hydrocarbons. The selectivity for theformation of butenes and butadiene is substantially as good as thatattainable at equivalent conditions in the absence of the Chydrocarbons. A stream of unsaturated C hydrocarbons and a stream ofisoprene are separated as products of the process.

The technical subject matter pertinent to the present invention isfurther clarified by reference to a plurality of examples.

EXAMPLE I An adiabatic dehydrogenation apparatus for laboratorypreparation of butadiene by the Houdry method can be described ascomprising a flowmeter measuring the amount of hydrocarbon fed to aheater, a catalyst chamber containing heat retention material and Houdrycatalyst (approximately 20% chromia on a sorptive alumina carrier), avacuum system maintaining an absolute pressure of about 177 mm. ofmercury in the catalyst chamber, a quenching system to cool quickly theetlluent from the catalyst zone, and a recovery system for isolating allof the products of the reaction. During the periodic regeneration, thecombustion of the carbonaceous deposit on the catalyst provides heat tothe catalyst bed, but the dehydrogenation portion of the cycle cools thecatalyst bed. In maintaining heat balance between the dehydrogenationand regeneration portions of the cycle, the temperatures of the inlet,middle and outlet portions of the bed are recorded, and average bedtemperatures are determined and utilized in evaluating runs. Thedehydrogenation results may be influenced by factors such as the extentof conversion, pressure, space rate, temperature, and the surface areaof the catalyst. The iso C products are best described as a percentageby weight of the corresponding iso C feed, without regard to whether theiso C hydrocarbons are pure or mixed with other gases. The products fromthe dehydrogenation of C hydrocarbons are also best described as apercentage of the C hydrocarbon feed. Although the same proportion ofbutane is converted to butadiene in the presence or absence of iso Chydrocarbons, the actual amount of butane subjected to dehydrogenationis less when iso C hydrocarbons are admixed, thus producing a totalquantity of butadiene smaller than in the absence of iso C hydrocarbons.

A mixture consisting of 25% isopentane and 75% normal butane was passedthrough the adiabatic dehydrogenation unit at 177 mm. Hg absolutepressure to simulate the conditions which might be employed inoncethrough operation for manufacture of butene as the principal productplus butadiene, isoprene and isopentenes as supplemental products. Theresults were compared with a run for a similar dehydrogenation of 100%normal butane. The results are indicated in Table I.

Wt. Percent Conv. of n0 H Net Butene, wt. Percent of nC4H1n. Butadiene,Wt. Percent of nC4Hl0- Total Selectivity, Percent Bed Inlet, F Bed Avg,F Bed Outlet, F Percent Oonv. of i051 Net Isopentene, wt. Percent ofiCfiHla Isoprene, wt. Percent of iC Hn Total i0 Select., Percent---Attention is called to the fact that in the improved method the extentof iso C hydrocarbon conversion was 36% and to the fact that the totaliso C selectivity was 75 This advantageously high total selectivity fordehydrogenation of isopentane to form isoprene and and isopentenes makesthe mixed C -C hydrocarbon dehydrogenation process attractive. Moreover,16% of the isopentane was dehydrogenated to isopentenes and 11% of theisopentane charge was converted to isoprene by the improved method. Theconversion of isopentane to isoprene is achieved more satisfactorily ina zone in which a significant amount of normal butane is also beingdehydrogenated than in the absence of such normal butane. Surprisingly,however, the presence of the isopentane undergoing dehydrogenation inthe same zone does not significantly impair the effectiveness of theprocess for converting normal butane to butene and/or butadiene. Thepercentage of butene obtained, the proportion of butadiene obtained andthe extent of conversion achieved are all substantially the same whetherthe isopentane dehydrogenation is or is not conducted simultaneously inthe same dehydrogenation zone.

EXAMPLE II In many commercial dehydrogenation units, a portion of theproduct gas is recycled through the dehydrogenation unit. A syntheticmixture comprising normal butane and recycle butenes, simulating thetype mixture sent to an industrial dehydrogenation unit, is passedthrough the previously described adiabatic apparatus for the laboratorystudy of dehydrogenation reactions. In the control ru the feedstockconsists of normal butane as fresh feed and recycle C hydrocarbons. Asimilar mixture, combined with recycle 0;, hydrocarbons, plus freshisopentane is passed through the catalytic zone in the improved method,thereby obtaining the data shown in Table II.

Table II Data Control Improved Method F tool! 100% 0 Es... 25% 0 's,

- 75% O 4's. Catalyst Surf. Area 27 m /g. Liquid Hourly Space Velocity 13 1.6 Butadiene, wt. percent of 04's 12. Recycle Butenes, Wt. percent ofO is 28. Butadiene Select. percent 55. Bed Inlet, F 1,095. Bed Avgz, F1,065. Bed Outlet, F 1,140. Wt. percent Conv. of i0 rs 18. 9. NetIsopentene, Wt. percent of i0 5S -3. 2. Isoprene, wt. percent of i0 sS15. 8. Ultimate Isoprene Select. percent 71. 5.

The conditions employed permitted the same percentage yield of butadieneto be produced in both methods. As the conditions employed in the runfor the improved method, 15.8% of the iC hydrocarbons charged to thecatalytic zone were recovered as isoprene. By reason of the 71.5%selectivity for isoprene at 18.9% iso C conversion, the method issuperior to some competitive procedures available for convertingisopentane to isoprene.

EXAMPLE III A laboratory unit for the dehydrogenation of hydrocarbonshaving fewer than 6 carbon atoms per molecule is adapted to permit therecycling of the portion of the product which is not withdrawn by theseparation procedures. The apparatus is operated continuously withvariations in the proportions of the feedstock introduced to the system.Operating data permit the calculation of estimated costs of productionof isoprene in a commercial plant. The necessity for recovering Cproducts involves both higher equipment costs and higher operating coststhan involved in a process from which only C, hydrocarbons arerecovered, and all such marginal costs must be carried by the isolated Cproducts. When a mixture consisting of by weight C hydrocarbons and 5%by weight C hydrocarbons is subjected to the standard dehydrogenationconditions, the quantities of isoprene produced are sufficiently smallthat the cost of recovery of the isoprene is excessive. In treating amixture of 65% by weight C hydrocarbons and 35% by weight Chydrocarbons, the selectivity for isoprene production from iso Chydrocarbons so nearly approaches the selectivity achieved in theabsence of C hydrocarbons that the savings attributable to themarginally superior selectivity are insuflicient to compensate for theequipment costs and operating costs for mixed dehydrogenation. It isestablished that the dehydrogenation of the mixture of normal C and isoC hydrocarbons by the method of the present invention must be conductedusing a mixture within the range from to 60% by weight iso Chydrocarbons and from 90% to 40% by weight normal C hydrocarbons.

EXAMPLE IV A gas mixture was prepared consisting of:

Percent by weight Ethane 0.1 Propane 0.1 Isobutane 1.9 N-butane 59.3Isopentane 31.8 Isopentenes 5.9 N-pentane 0.8 N-pentenes 0.1 Total C38.6

The gas mixture was passed through a catalytic dehydrogenation zone atconditions of 1.2 LHSV, 177 mm. Hg absolute pressure, over a bedcontaining 40% alumina granules and 60% chromia-alumina catalyst havinga surface area of 25 m.*/ g. to convert 36% of the butane at a totalselectivity of 65%. Simultaneously, 24% of the iso C hydrocarbons wereconverted at 90 total selectivity. The isoprene yield per pass was Thetemperature was raised to convert 48% of the butane at a totalselectivity of 62%, and at the same time, 29% of the iso C hydrocarbonswere converted at a total selectivity of 76%. The isoprene yield perpass was 18%.

Not only is the dehydrogenation of the iso C hydrocarbons more selectivein the presence of normal C hydrocarbons undergoing dehydrogenation butalso it is feasible to convert a higher percentage of the iso Chydrocarbons without encountering difiiculties in maintaining aheat-balanced operation. Higher conversions per pass permit lowerrecycle ratios, thereby permitting smaller units (involving less capitalinvestment) for a given capacity to produce isoprene. The combination ofhigher conversion and higher selectivity for isoprene production whilestill utilizing the Houdry method of frequently regenerating thecatalyst makes attractive the conversion of butadiene plants to plantsproducing both butadiene and isoprene.

EXAMPLE V A mixture of 40% by weight butane and 60% isopentane is passedthrough a bed containing 40% heatretentive inert alumina granules and60% chrornia-alumina dehydrogenation catalyst at a liquid hydrocarbonspace velocity of 1.3 volumes per volume of catalyst per hour at 1075 F.at 127 mm. Hg absolute pressure to obtain 45% conversion of theisopentane at a total selectivity of 73%. The isoprene yield per pass is12% and the piperylene yield is only 2.5%, providing an advantageousratio of almost 5 to 1.

EXAMPLE VI Several mixtures of normal C and iso C hydrocarbons wereprepared and stored in tanks, the mixtures simulating variouscombinations of once-through and recycle processing. Each mixtureconsisted of about 75% normal C hydrocarbons and about 25% iso Chydrocarbons.

Sample A represented a mixture containing both recycle normal Chydrocarbons and recycle iso C hydrocarbons. If only the normal Chydrocarbons were to be recycled, the composition might be like B and ifonly the iso C hydrocarbons were recycled, the composition might be likeC. Sample D represented once-through feedstock. Each of the mixtures (A,B and C) simulating a mixture containing a recycle stream contained moresaturated than olcfinic hydrocarbons. Commercial operation has indicatedthat in dehydrogenation units having a recycle stream, normal butane isthe most abundant nC hydrocarbon component in the total feed if normalbutane is the most abundant nC hydrocarbon in the fresh feed, and suchrelationships would be expected to prevail in the dehydrogenation ofmixtures of C and C hydrocarbons.

A dehydrogenation unit containing 40% inerts, 60% catalyst(chromia-alurnina aged to a surface area of 27 m. g.) particles wasemployed adiabatically on a oncethrough basis. The dehydrogenation wasconducted at 177 mm. Hg absolute pressure on a 9 minute dehydrogenation,9 minute regeneration, 3 minute purge cycle. Data relating to severalruns are shown in Table III. The abbreviation NC is employed to indicatethat the information was not available for some reason such as forexample, the value was not calculated or the value was not measured.

Table III Feedstock A B C D L' uid Hourly Space Velocity 1.6 1. 7 1. 21.2 Inlet. F 1,072 1,124 1,108 1,099 Average, F. 1,052 1,080 1,058 1,055Outlet, F a 1,100 1,154 1,088 1, 080 04113, wt. Percent of C4Feed. 6. 44. 0 19. 5 20. 9 CAHG, wt. Percent of OilFeed... 10. 5 12. 6 6.8 6.6Percent C4 Conversion 13. 2 28.2 35.9 38. 6 Total C; Selectivity, PercenNO N O 74 71 Ultimate C4Hs Selectivity Per 54 52 NO N 0 10 E10, wt.Percent 0110 Feed -0. 5 13. 9 0.5 16. 4 Isoprene, wt. Percent of 0 Feed14. 9 10. 9 17.6 10. 7 Percent 05 Conversion 15. 3 40. 9 22.6 36.0 Total0 Selectivity, Percent N G 01 75 Ultimate Isoprene Selectivity, Percent94 NC 79 NC A comparison of such data with comparable runs which werenot C -C mixtures established that the presence of the C hydrocarbonshad no significant effect upon the dehydrogenation of the C hydrocarbonsbut that the presence of the C hydrocarbons had an important influenceupon the C dehydrogenation. The isoprene to piperylene unit ratios whenthe dehydrogenation is conducted in the presence of normal Chydrocarbons are as high as 11, in contrast with unit ratios such as 2or 3 in the absence of such normal C hydrocarbons. The presence ofnormal C hydrocarbons permits operation of the isopentanedehydrogenation step at higher temperatures While still restricting cokeproduction to commercially satisfactorily low levels.

By a series of tests, it is established that the limits for the methodare the inclusion of at least 10% but not more than 60% by weight iso Chydrocarbons in the mixture of normal C and iso C hydrocarbons, themaintenance of a pressure less than 15 inches of mercury, the use of adehydrogenation-regeneration cycle maintaining heat balance in theapproximately adiabatic dehydrogenation, the maintenance ofdehydrogenation severity sufficient to convert at least 10% of the Chydrocarbons, the maintenance of an average bed temperature of at least1050 F. and less than 1200 F. and a space velocity of at least 1 butless than 6 volumes of liquid hydrocarbon per volume of catalyst per bedhour. Because the dehydrogenation of C -C hydrocarbon mixtures inaccordance with the present invention permits adiabatic operation athigher conversion levels and with better isoprene selectivity than ispossible in the dehydrogenation of isopentane, there is more than ampleeconomic justification for the relatively expensive separation systemnecessary for recovering isoprene and butadiene from the effluent fromthe catalytic dehydrogenation zone.

-'It should be noted that the method of the present invention involvesthe treatment of a hydrocarbon gas stream by passage through anadiabatic bed of granules of dehydrogenation catalyst at a space rate ofabout 1 to 6 liquid volumes per catalyst volume per hour at atemperature within the range between 1050 F. and 1200 F. at an absolutepressure lower than about 15 inches of mercury to produce an efilnentcomprising hydrogen, C -C hydrocarbons, a product stream of unsaturatedC hydrocarbons, a product stream of isoprene, a C hydrocarbon recyclestream, and a C hydrocarbon recycle stream. A particularly importantfeature of the present invention is the attainment of better selectivityin the conversion of C hydrocarbons to isoprene than would be obtainedat equivalent conditions in the absence of the C hydrocanbons. Thisadvantageous result of improved selectivity is achieved by controllingthe composition of the gas stream directed to the catalyst bed so thatthe weight concentration of the C hydrocarbons is within the rangebetween 40 and 90% and the weight concentration of iso C hydrocarbons iswithin the range between and 60%, normal butane being the most abundantcomponent of the C hydrocarbons, and isopentane being the most abundantcomponent of the C hydrocarbons in said gas stream.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claim.

The invention claimed is:

The method of preparing isoprene and butadiene simultaneously whichincludes the steps of:

directing C hydrocarbons from a source of supply as a stream of fresh Chydrocarbons toward a mixing zone;

directing C hydrocarbons from a source of supply as a stream of fresh Chydrocarbons toward a mixing zone;

directiong C hydrocarbons from a hereinafter designated separation zoneas a stream of recycled C hydrocarbons toward a mixing zone;

directing C hydrocarbons for a hereinafter designated separation zone asa stream of recycled C hydrocarbons toward a mixing zone; preparing ahydrocarbon gas stream in the mixing zone consisting essentially of amixture of said:

(1) recycled C hydrocarbons, (2) recycled C hydrocarbons, (3) fresh Chydrocarbons, and (4) fresh C hydrocarbons, said gas stream consistingof from 40% to 90% by weight normal 0.; hydrocarbons and from 10% to isoC hydrocarbons, normal butane being the most abundant component of the Chydrocarbons and isopentane being the most abundant component of the Chydrocarbons in said gas stream; directing said hydrocarbon gas streamthrough chromia on alumina catalyst at a space rate of about 1 to 6liquid hydrocarbon volumes per volume of catalyst per hour at anabsolute pressure less than 15 inches of mercury at a temperature Withinthe range between 1050 F. and 1200 F.; directing the efiiuent from thecatalyst bed to a separation zone in which the efliuent is separatedinto a plurality of streams, including a stream of recycled Chydrocarbons, a stream of recycled C hydrocarbons, and product streams;separating in said separation zone as a product of the method a streamof unsaturated C hydrocarbons, the quantity of butadiene constituting aconversion of the normal butane to butadiene substantially as good asthat attainable at equivalent conditions in the absence of the Chydrocarbons; and separating as a product of the method a stream ofisoprene, the quantity of isoprene constituting a conversion of theisopentane to isoprene better than attainable at equivalent conditionsin the absence of the C hydrocarbons.

References Cited in the file of this patent UNITED STATES PATENTS2,401,973 Seyfried et a1. June 11, 1946 2,440,492 Seyfried et a1. Apr.27, 1948 2,458,082 Kilpatrick Jan. 4, 1949 2,900,429 Heinemann et a1Aug. 18, 1959

